1. Field of the Invention
This invention relates to a valve assembly for use in an engine.
2. Description of Related Art Including Information Disclosed Under 37 C.F.R. 1.97 and 1.98
Engine valves control fluid flow into and out of an engine cylinder or combustion chamber. They fit into the cylinder head and operate inside valve guides. Valve springs fit over the top end of the valves to keep the valves in a normally closed position. Conventionally, each valve has a valve face, valve seat, margin, stem, and a tip end. When slid down, the valve slides away from its seat and the port is opened. When slid upwardly, the valve makes contact with its seat to seal the combustion chamber from the port.
The intake valve is often a larger valve that allows a fuel charge to flow into an engine cylinder. Typically, an air-fuel mixture flows through the intake port, past the valve, and into the combustion chamber when the valve is opened. The exhaust valve may be a smaller valve that opens to allow burned gases to escape from the engine.
Automotive engines, both gas and diesel, are normally four-stroke cycle engines. The four strokes are the intake stroke, compression stroke, power stroke and the exhaust stroke. During the intake stroke, air and fuel are drawn into the combustion chamber. The piston slides downwardly to create a vacuum. The intake valve is opened, and the exhaust valve is closed. Thus, the cylinder becomes filled with an ignitable mixture of fuel and air.
During the compression stroke, the air-fuel mixture is squeezed to make it more combustible. Both the intake and exhaust valves are closed. The piston slides upwardly, and compresses the mixture into a small area of the combustion chamber. For proper combustion, it is important that the valves, rings, and other components do not allow pressure leakage after the combustion chamber. Leakage would keep the mixture from burning and igniting on the power stroke. During the power stroke, the air-fuel mixture is ignited and burned to produce gas expansion, pressure, and a powerful downward piston movement. Both valves are closed. In a spark ignited engine, a spark plug initiates the fuel mixture combustion. During burning, the mixture expands and pressure accumulates in the combustion chamber. Since the piston is the only movable part, it is thrust downwardly. The downward movement is communicated to a connecting rod and crank shaft, which is forced to rotate.
An exhaust stroke expels the burned gas from the cylinder and into the car's exhaust system. The intake valve remains closed, and the exhaust valve slides open. Since the piston is now moving upwardly, burned fumes are expelled from the exhaust port to prepare the cylinder to receive a fresh charge of a combustible air-fuel mixture. During the exhaust stroke, there continues to be a need for a sealing engagement between the intake valve and its seat, even in the advanced phases of the engine's service life.
Conventionally, valve seats are round, machined surfaces received in the port openings to the combustion chambers. When the engine valve closes, the valve touches the seat to seal the port. The valve seats can be part of the cylinder head, or be formed as a separate pressed-in component. An integral valve seat is made by using a tool to machine a precise face on the port opening into the combustion chamber. The seat is aligned with and centered around the valve guide so the valve centers on the seat. A pressed-in valve seat or a seat insert is typically a separate machined part which is press-fitted into the cylinder head. The recess defined into the combustion chamber is slightly smaller than the OD of the insert. A press is used to drive the insert into the head. Friction retains the seat in relation to the head.
Typically, steel valve seat inserts are used in aluminum cylinder heads. Steel is needed to withstand the high operating temperatures produced by combustion.
In gasoline engines, a seat insert is not commonly used in cast iron cylinder heads because heat is not dissipated as quickly as with integral seats. In heavy duty diesel engines, low or high alloy inserts may be used in cast iron heads.
The characteristics of hardness and resistance to wear are often imbued by induction hardening which is conventionally engendered by an electric-heating operation. Induction hardened valve seats may be used in engines to increase service life, although many late model engines include aluminum cylinder heads in which valve seats cannot readily be induction hardened.
Lead additives in fuel have historically helped lubricate the contact between the valves and the valve seats. At high temperatures, the lead acts as a lubricant therebetween, but unleaded fuel today lacks leaded lubricants. Additionally, engine operating temperatures tend to be higher. Thus, the problems of valve and valve seat wear become more pronounced. To withstand these challenging conditions, hardened valve faces and seats, especially on exhaust seats, are required.